In a radio frequency (RF) transmitter comprising an oscillator and a power amplifier, oscillator remodulation can occur because of RF coupling between the power amplifier and the oscillator. This coupling is particularly likely to occur in transmitters where the power amplifier and the oscillator are manufactured in the same integrated circuit (IC) or in the same device package. As a result of the coupling, a fraction of the output power is injected back into the oscillator circuit as an unwanted parasitic signal. This parasitic signal can disturb the operation of the oscillator, especially if the parasitic signal has spectral content close to the oscillation frequency, where the oscillator is particularly sensitive. In response to such a close-frequency perturbation, the oscillator will be phase or frequency modulated. This phenomenon is known as remodulation. Remodulation is more likely to happen in transmitters in which the oscillator runs at the fundamental transmit frequency or at an integer multiple of that frequency. If the oscillator runs at the fundamental transmit frequency, it is the fundamental frequency of the amplifier output signal itself that causes remodulation, whereas if the oscillator runs at n times the transmit frequency, remodulation is caused by the n-th harmonic of the amplifier output signal.
RF transmitters using a polar architecture are becoming popular in modern wireless communications systems. For example, such polar transmitters are used in mobile telephones conforming to the GSM and EDGE standards, but it is anticipated that the polar architecture will increasingly be used in products conforming to other standards such Bluetooth, DECT, UMTS and other 3G cellular standards, WLAN and Wimax.
Polar transmitters, in which the carrier frequency follows that of the oscillator by construction, are less subject to oscillator remodulation than Cartesian transmitters. However remodulation can nevertheless happen in a polar transmitter due to amplitude variations of the output signal, as occurs with non-constant envelope modulation.
One form of polar transmitter is a “direct” polar transmitter which does not have a feedback loop to correct for imperfections in the amplitude and frequency paths of the transmitter. A typical example of “direct” polar transmitter architecture is described in “A Quad-Band 8PSK/GMSK Polar Transceiver”, Hietala, A. W., IEEE Journal of Solid-State Circuits, vol. 41, no. 5, May 2006, pp. 1133-1141.
An object of the invention is to reduce remodulation in a wireless transmitter.